News from ‘El Gordo’: Study suggests dark matter may eventually have collisional properties

Contrary to what is determined by the standard model, dark matter can indeed interact on its own. This was the conclusion of a study published in Astronomy and Astrophysics and directed by Riccardo Valdarnini of SISSA’s Astrophysics and Cosmology group. Using numerical simulations, the study analyzed what happens inside “El Gordo” (literally “The Fat One” in Spanish), a giant merger cluster seven billion light years away.

Calculations showed that in this group the observed physical separation between the points of maximum density of dark matter and those of other mass components can be explained using the so-called SIDM (Self-Interacting Dark Matter) model, compared to the standard one. one. This research makes an important contribution in favor of the SIDM model, according to which dark matter particles exchange energy through collisions, with interesting astrophysical consequences.

‘El Gordo’: A giant cosmic structure for the study of dark matter

“According to the currently accepted standard cosmological model, the actual baryonic matter density of the universe may only constitute 10% of its total matter content. The remaining 90% is in the form of Dark Matter,” explains Valdarnini, author of the research .

“This matter is generally thought to be non-baryonic and composed of cold collisionless particles that respond only to gravity. Hence the name ‘Cold Dark Matter’ (CDM). However, there are still a number of observations that have not been made still. explained using the standard model,” says the researcher. “To answer these questions, some authors suggest an alternative model, called SIDM.”

Testing the collisional properties of dark matter, and, more generally, alternative theories to the standard cosmological model, is very complicated. He explains, “There are, however, unique laboratories that can be very useful for this purpose, many light years away from us. These are the massive clusters of galaxies, the gigantic cosmic structures that, after the collision, determine the most energetic events since the Big Bang.

“By a measure of about 10¹⁵ solar masses, El Gordo is one of the largest galaxy clusters known. Due to its features, El Gordo has been the subject of numerous theoretical and observational studies.”

Dark matter can be collisions

According to the standard paradigm, during an accretion merger, the behavior of the gas mass component in the collision will differ from that of the other two components – galaxies and dark matter. In this scenario, the gas will dissipate some of its initial energy.

“This is why, after the collision, the peak of the gas mass density will lag behind those of dark matter and galaxies,” explains Valdarnini. With the SIDM model, however, a special phenomenon should be observed, namely the physical separation of the centroids of dark matter – the points of its maximum density – from those of other mass components with features that represent a “Signature of SIDM models”. . According to observations, this is exactly what happens inside “El Gordo”.

Observing El Gordon

“Let’s start with the observations,” explains Valdarnini. El Gordo consists of two massive subgroups, respectively named northwest (NW) and southeast (SE). The X-ray image of the El Gordo cluster shows a single X-ray emission peak in the SE subcluster and two faint tails extending beyond the X-ray peak. An important feature is the peak location of the various mass components .

In contrast to what can be seen in the bullet cluster, another important example of a collision cluster, the X-ray peak precedes the SE dark matter peak. Moreover, the Brightest Cluster Galaxy (BCG) is not only following the X-ray peak, but also appears to have moved in space from the SE center of mass. Another notable feature can be seen in the VP cluster, where the galaxy number density peak is spatially displaced from the corresponding mass peak.

Research findings suggest dark matter collision as an explanation for phenomena observed at ‘El Gordo’

To explain his findings and validate the SIDM models, Valdarnini used a large set of so-called N-body/hydrodynamic simulations. Thus, he conducted a systematic study aimed at reproducing the observational features of “El Gordo”.

“The most significant result of this simulation study is that the observed relative separations between the different centers of mass of the ‘El Gordo’ cluster are naturally explained if dark matter is self-interacting,” says Valdarnini.

“Therefore, these findings provide a clear signature of a behavior of dark matter exhibiting collisional properties in a high-energy, high-redshift cluster collision. However, there are inconsistencies, as SIDM cross-section values ​​obtained from these simulations are higher than present upper bounds, which are of the order of unity on cluster scales.

“This suggests that current SIDM models should only be considered as a low-order approximation and that the underlying physical processes describing the interaction of dark matter in major cluster mergers are more complex than can be adequately represented by the commonly assumed approach based on the distribution of dark matter particles.

“The study makes a compelling case for the possibility of dark matter self-interaction between colliding clusters as an alternative to the standard collisionless dark matter paradigm.”